Great questions!

I run them overclocked for the extra heat. They run slightly less efficiently this way but it doesn't matter since the house needs to be warm. I also want to stack the max amount of sats I can before the halvening. Once that hits I will run them as efficiently as possible since the heat will be exhausted anyways. The overclock setting I'm using now is built into the stock firmware so I'm thinking it's within the machines capabilities.

As far as the noise, I ran them stock while building the system. It was equivalent to about 2 vacuum cleaners running all the time. Now that they are immersed, the only sounds are the pumps and they are essentially silent!